Recently, display devices have been remarkably researched and developed, and a thin flat panel display (FPD) has been in widespread use instead of a display device employing a cathode-ray tube which was conventionally in widespread use. The FPD includes a display element such as liquid crystal, a light emitting diode (LED), or organic electroluminescence (EL).
The FPD is configured such that light is emitted to a display screen, or light is emitted by, for example, a backlight which is provided behind the display screen (which is provided so as to face a viewer via the display screen). The viewer views the light emitted from the display screen. The display device is designed such that a viewer can view light emitted obliquely from the display screen in the same manner as light emitted straightforwardly from the display screen. That is, the display device is designed such that the viewer does not feel difference between when the viewer obliquely views the display screen and when the viewer straightforwardly views the display screen. However, the design is insufficient. There is a case where a viewer feels a great difference between when the viewer obliquely views the display screen and when the viewer straightforwardly views the display screen though a contrast property is excellent when the display screen is straightforwardly viewed. Therefore, display on the display screen differs depending on a direction in which the viewer views the display screen. That is, the FPD has a problem of deterioration in viewing angle property.
In order to improve the viewing angle property of the FPD, for example, a light diffusing film is provided on a light exit surface of the FPD. This makes it possible for a viewer to view an image in an oblique direction. Examples of the light diffusing sheet include a film whose surface is concavo-convex, and a film containing light diffusing fine particles. The light diffusing sheet subjects light emitted from the backlight to refraction or total reflection in all directions by use of difference in refractive index. The light refracted by the light diffusing sheet is diffused from the surface of the light diffusing sheet in many directions, and is emitted toward a viewer. Usage of the light diffusing sheet makes it possible for a viewer to view an image on the display device from various directions. It is therefore possible to develop a display device which displays an image which (i) does not differ from a case where the image is straightforwardly viewed to a case where the image is obliquely viewed and (ii) less changes depending on a viewing angle.
For example, Patent Literature 1 discloses a display device including a light diffusing sheet provided on a side of the display device which side is to be viewed. In the light diffusing sheet, a plurality of grooves are formed in juxtaposition with one another, each of which grooves has a substantially V-shaped cross section. FIG. 11 illustrates the display device including the light diffusing sheet disclosed in Patent Literature 1. As illustrated in FIG. 11, a light diffusing sheet 20 of a display device 11 disclosed in Patent Literature 1 has formed a plurality of waveguides 17 any adjacent ones of which are separated by a corresponding one of a plurality of substantially V-shaped grooves (air gap regions) 26. Each of the plurality of waveguides 17 has an incident surface and a light exit surface that is distal to the incident surface. The incident surface has a surface area larger than that of the light exit surface. The waveguide 17 has a tapered shape which tapers such that the light exit surface has a surface area smaller than that of the incident surface. Therefore, angle distribution of light emitted from the light exit surface is broader than that of light which enters the incident surface. As such, according to the technique disclosed in Patent Literature 1, it is possible to realize the display device 11 having an excellent viewing angle property, and high contrast and vividness.